docs: def _apply_summaries(self):
          """Add all summary rows and columns."""
  
          def as_frame(r):
              if isinstance(r, pd.Series):
                  return r.to_frame()
              else:
                  return r
  
          df = self.data
  
          if df.index.nlevels > 1:
              raise ValueError(
                  "You cannot currently have both summary rows and columns on a "
                  "MultiIndex."
              )
  
          _df = df
          if self.summary_rows:
              rows = pd.concat([agg.apply(_df)
                                for agg in self._cleaned_summary_rows], axis=1).T
              df = pd.concat([df, as_frame(rows)], axis=0)
  
          if self.summary_cols:
              cols = pd.concat([agg.apply(_df)
                                for agg in self._cleaned_summary_cols], axis=1)
              df = pd.concat([df, as_frame(cols)], axis=1)
  
          return df